1. Field of the Invention
Embodiments of the invention relate to the field of semiconductor device fabrication. More particularly, the present invention relates to an apparatus and method for controlling the temperature of an ion source within an ion implanter.
2. Discussion of Related Art
Ion implantation is a process used to dope impurity ions into a semiconductor substrate. An ion beam is directed from an ion source chamber toward a substrate. The depth of implantation into the substrate is based on the ion implant energy and the mass of the ions generated in the source chamber. A precise doping profile in the substrate is critical to proper device operation. One or more types of ion species may be implanted in different doses and at different energy levels to obtain desired device characteristics. FIG. 1 is a block diagram of an ion implanter 100 including an ion source chamber 102. A power supply 101 supplies the required energy to source 102 which is configured to generate ions of a particular species. The generated ions are extracted from the source through a series of electrodes 104 (extraction electrode assembly) and formed into a beam 95 which passes through a mass analyzer magnet 106. The mass analyzer is configured with a particular magnetic field such that only the ions with a desired mass-to-charge ratio are able to travel through the analyzer for maximum transmission through the mass resolving slit 107. Ions of the desired species pass from mass slit 107 through deceleration stage 108 to collimator magnet 110. Collimator magnet 110 is energized to deflect ion beamlets in accordance with the strength and direction of the applied magnetic field to form a ribbon-shaped beam targeted toward a work piece or substrate positioned on support (e.g. platen) 114. In some embodiments, a second deceleration stage 112 may be disposed between collimator magnet 110 and support 114. The ions lose energy when they collide with electrons and nuclei in the substrate and come to rest at a desired depth within the substrate based on the acceleration energy.
An indirectly heated cathode (IHC) ion source is typically used as the ion source chamber 102 in high current applications. FIG. 2 is a cross sectional block diagram generally illustrating an IHC ion source 200 including an arc chamber 201 defined by electrically conductive (e.g. tungsten) chamber walls. The chamber defines an ionization zone within which energy is imparted to a dopant feed gas to generate associated ions. Different feed gases are supplied to the ion source chamber to obtain plasma used to form ion beams having particular dopant characteristics. For example, the introduction of H2, BF3 and AsH3 as the dopant gas at relatively high chamber temperatures are broken down into mono-atoms having high implant energies.
The IHC ion source chamber 200 includes a cathode/filament assembly 230 located at one end of the arc chamber 201. A filament 231 is positioned in close proximity to cathode 232 outside the arc chamber 201. A voltage is supplied to filament 231 which produces enough current through the filament to heat it and cause thermionic emission of electrons. Cathode 232 is indirectly heated via filament 231 by biasing the cathode more positively than the filament which causes these thermo-electrons to accelerate from filament 231 toward cathode 232, thereby heating the cathode 232. A repeller 210 is typically positioned on the opposite end of the arc chamber 201 and is biased to the same voltage as cathode 232. The emitted electrons are confined between the cathode 232 and repeller 210 which collide with the dopant feed gas introduced into the chamber via conduit 212 to generate plasma having the desired properties.
The ions 222 formed from the dopant gas are extracted from source chamber 200 via aperture 220 by way of, for example, a standard three (3) electrode configuration comprising plasma electrode 215, suppression electrode 216 and ground electrode 217 used to create an electric field. Although suppression electrode 216 is shown as being spaced apart from ground electrode 217, this is for illustrative purposes only and the electrodes are physically in contact with each other via insulators. Plasma electrode 215 may be biased at the same large potential as ion source chamber 200. Suppression electrode 216 is connected to a power supply and is typically biased at a moderate negative value to prevent electrons from entering back into source chamber 200. Ground electrode 217 is positioned downstream from suppression electrode 216 and is at ground potential. The strength of the electric field generated by the electrodes can be tuned to a desired beam current to extract a particular type of ion beam from the ions 222 generated in chamber 200.
FIG. 2A is a cross section of ion source 200 taken along lines A-A. Faceplate 262 includes aperture 220 through which beam 222 is extracted using extraction electrode assembly including suppression electrode 216, ground electrode 217 (and plasma electrode) as described above. The arc chamber 201 includes liners 250 disposed along sidewalls 260 and endplate 261. The walls of the chamber and the liners define a gap therebetween through which dopant gas, supplied via conduit 212, enters the chamber 201. These liners provide a low-cost consumable part that may be replaced as well as providing a uniform distribution of the dopant gas into the arc chamber, thereby providing more uniform and stable ion source operation. However, due to excessive source operation and the fact that these liners are thermally isolated from the walls of the arc chamber, the liners may become overheated. As a result, excessive sputtering and/or chemical etching causes particle generations which stick or deposit on the liners.
These same IHC ion sources may also be used for high-current (e.g. >100 mA extraction current) phosphorus implantations which require that the IHC ion source run at cooler-than-normal source temperatures to improve beam current and provide more stable implant profiles. Operating the IHC ion source at cooler-than-normal source temperatures also improves fractionization of the phosphorus ion species. However, the particles generated from the excessive sputtering and/or chemical etching may cause unstable ion source operation and beam extraction thereby compromising the desired beam profile. One alternative to overcome these problems is to utilize the source chamber without the liners, thereby making the source chamber somewhat cooler by eliminating thermally isolated hot spots. However, these thicker walls do not provide temperature-control capability for high current and high throughput operations. Thus, there is a need for an ion source that can be operated at a desired temperature for stable, high throughput ion implantations. In addition, there is a need to control the temperature of an ion source chamber by utilizing the same dopant gas for both temperature-control and dopant species generation.